1. Field of the Invention
This invention is generally related to an actuator for operating a wellhead choke using a planetary roller screw. More particularly, the invention provides apparatus for operating a choke in a subsea environment that performs the fast close function of a hydraulic motor while giving control similar to that of a stepping actuator in a single compact unit.
2. General Background
The ongoing search for hydrocarbons in offshore locations to satisfy the world""s increasing need for energy has led to the need to drill for and produce these hydrocarbons in increasingly deeper waters. This has led to the need for production trees suitable for use in a subsea environment. These subsea trees require valves and chokes that can be operated remotely with a minimum of diver intervention. In the case of deep water trees where diver intervention is not possible, the only alternative is to use remotely operated vehicles (ROVs) to provide intervention capabilities. Therefore, it is necessary that any such actuator for a choke or valve be operable by normal hydraulic means while allowing for override by an ROV.
Current technology often uses a hydraulic torque motor when it is desired to have a fast closing operation. These hydraulic torque motors suffer from a number of deficiencies. Among these are the motors require a large amount of hydraulic fluid to operate and the amount and type of fluid required may vary according to the motor design and the required output torque for varying loads, Additionally, the running torque is much higher than the starting torque which can cause problems at stall conditions at the end of the travel. The roller screw actuator of the present invention overcomes these deficiencies.
2. Description of Related Art
U.S. Pat. Nos. 4,741,250; 4,881,419; and 5,027,667 to P. P. Weyer show hydraulically and spring powered actuators using various rollers.
A high torque rotating actuator suitable for subsea use is disclosed in U.S. Pat. No. 6,231,027 B1 to G. S. Baker et al.
The present invention comprises an actuator for use with a subsea choke or similar apparatus. The actuator uses a planetary roller screw mechanism to allow greater control of the torque output while allowing a fast closing of the choke when well conditions require so. The actuator includes a two part housing secured in abutting relationship by bolts. Centrally disposed in the actuator housing is a driven or output shaft that is supported in support assemblies at each end. These support assemblies include support and thrust bearings that restrain the driven shaft from axial movement while allowing rotation. A handle is positioned on one end of the actuator housing for manipulation of the actuator housing by a remotely operated vehicle, when required. A plurality of anti-rotation keys are positioned on the opposite end of the actuator housing. A pressure responsive axially reciprocable piston is positioned around the driven shaft. The piston has a counterbore and a bore therethrough.
A plurality of spindle shafts are disposed circumferentially around the counterbore of the piston. The plurality of spindle shafts are helically threaded and in threaded engagement with a complementary helical thread on the central driven shaft. An anti-rotation means is disposed between the piston and the actuator housing to prevent rotation of the piston as it is reciprocated. The support assemblies positioned on each end of the central driven shaft are coupled to the ends of the central driven shaft by mating splines. The support assembly adjacent the end of the actuator housing supporting the plurality of anti-rotation keys includes an extension extending through a bore in the end of the actuator housing. The extension includes a splined end having a splined adapter thereon having an internal hex on its outer end.
The axially reciprocable piston includes a annular flange formed thereon that is positioned in a counterbore in the actuator housing. The annular flange and the counterbore have annular seals positioned thereon to form an annular chamber in which the piston reciprocates. The actuator housing includes pressure ports at either end of the annular chamber to supply pressurized fluid for reciprocation of the piston. Another embodiment showing the actuator controlling the position of a choke cage is shown along with the actuator as used on a subsea tree.
A principal object of the present invention is to provide an actuator for a choke that allows fast closing of the choke while giving control of the positioning of flow control element approximating that of a stepping actuator.
These with other objects and advantages of the present invention are pointed out with specificness in the claims annexed hereto and form a part of this disclosure. A full and complete understanding of the invention may be had by reference to the accompanying drawings and description of the preferred embodiments.